1. Field of the Invention
The present invention relates to the control of brushless motors, particularly in the system described in U.S. patent application Ser. No. 08/827,747, which is herein incorporated by reference.
2. Discussion of the Related Art
FIG. 1 shows a cross-sectional view of a typical brushless, DC motor. The motor includes a permanent magnet rotor 12 and a stator 14 having a number of windings (A, B, C shown in FIG. 2). The windings are disposed in a plurality of slots 18. The motor illustrated has rotor 12 housed within stator 14. Stator 14 may also be housed within rotor 12. Rotor 12 is permanently magnetized, and rotates to align its magnetic flux with the magnetic flux generated by the windings.
FIG. 2 shows an electrical diagram of the stator of such an electric motor, and of the supply control circuitry used. Often, such motors comprise three phases A, B, C. These may be connected in a star (xe2x80x98wyexe2x80x99) configuration having a common node N (as in the drawing), or in a delta configuration. The free end of each winding is connected by a pair of switches XSA, XGA; XSB, XGB; XSC, XGC to supply Vs and ground GND voltages, respectively. A reverse-biased diode DSA, DGA; DSB, DGB; DSC, DGC is placed in parallel with each of the switches. The opening and closing of the switches may be controlled by a microcontroller.
As shown in FIG. 3, the switches are controlled according to a sequence of steps. The diagram shows the voltages VA, VB, VC applied to each winding, relative to common node N. The drawing shows the case of a motor having phases A, B, C, controlled through six steps s1, s2, s3, s4, s5, s6, each corresponding to a specific magnetic flux pattern in the motor. In each of the six steps, one of phases A, B, C is off, and the other two are oppositely polarized. The rotor aligns its magnetic flux with that of the stator, and thus rotates synchronously with the stator flux, as it rotates due to the switching of the six steps.
When the rotor rotates, its rotation induces a back emf voltage in each of the windings of the motor. For a loaded motor, the back emf generated in a winding is approximately in phase with the current flowing in the same winding.
FIG. 4 shows the six steps of voltage VA applied across winding A, of back emf BemfA generated in winding A by the rotor under load, and of current IA through winding A. The periods of increasing current and decreasing current are respectively called the energizing period (pe) and the de-energizing period (pd). Extending between the de-energizing period and the energizing period is a surveillance period td, during which the back emf may be monitored.
The voltage effectively issued to each of the motor windings is controlled by pulse-width modulation (PWM) of the DC supply voltage. The frequency of the pulse-width modulation signal is usually high with respect to the rotating frequency of the motor, for example, of approximately 10 kHz. The switches are controlled by a microcontroller according to the current to be issued to the motor to ensure the switching between steps s1 to s6 and to ensure the PWM control.
During low level periods between PWM pulses, the motor turns in free wheel; the kinetic energy of the motor as it turns is transformed into electric energy by its rotation in a magnetic field. The motor does not slow down during these periods since the high values of the PWM frequency, of the motor inertia and of its load make these changes undetectable.
For the motor to operate properly, the flux existing in the stator must always be slightly advanced with respect to the rotor in order to pull the motor forward. Also, the flux in the stator just behind the rotor is advantageously of a polarity adapted to repelling the rotor to help the synchronization. However, if the synchronization between the rotor motion and the flux rotation is lost, the rotor may stop rotating or may operate with a poor efficiency. Accordingly, to optimize the motor efficiency, the switching of the windings from one step to another must be controlled according to the effective position of the rotor. For this purpose, in a self-commutated motor operating mode, the back emf induced in the windings and more specifically the zero crossing points of this back emf are monitored to determine the position of the rotor at a given time.
U.S. patent application Ser. No. 08/827,747 provides a control circuit of a brushless electric motor in which the transition from one step to the following is determined by a zero crossing of the back emf in a winding which is not supplied with a voltage during this step.
More specifically, U.S. patent application Ser. No. 08/827,747 provides a method of control of a brushless motor having a plurality of windings, each of which has a first end connected to a common node and a second end which can be connected to supply voltages, including the following steps:
a) applying an upper supply voltage to a second end of each of a first subset of windings, applying a lower supply voltage to a second end of each of a second subset of windings, the second end of at least one winding being disconnected from the supply voltages; the application of the supply voltage to at least one of the first and second subsets being periodically interrupted;
b) detecting the presence of a current in the disconnected winding, then detecting a stopping time of the current;
c) beginning at the stopping time, monitoring the value of a back emf induced in the disconnected winding and detecting a zero crossing point of the back emf with respect to the upper and/or lower supply voltage;
d) counting down a predetermined delay from the detected zero crossing point; and
e) after the end of the predetermined delay, removing the supply voltages from the second ends of the first and second subsets of windings and applying the upper supply voltage to a second end of each of a third subset of windings; applying the lower supply voltage to a second end of each of a fourth subset of windings; the second end of at least one winding remaining unconnected to the supply voltages.
According to U.S. patent application Ser. No. 08/827,747 the predetermined delay is a predetermined fraction of the duration taken to perform a previous cycle of steps b) to e), this predetermined fraction being likely to be modified during operation of the motor.
The applicant has noted that, in the case where the motor exhibits a magnet disymmetry or is magnetically deformed, or if the mechanical symmetry of the stator is not exactly the same as that of the rotor, the electric position information corresponding to the zero crossing of the back emf is not steady in time although the rotor speed is steady.
In the case where a delay T which is proportional to the duration of the directly preceding interval appears between zero crossings, an increased disymmetry appears.
The present invention aims at overcoming this disadvantage.
The present invention provides a self-commutated type method of control of a brushless motor having a plurality of windings switched according to a determined cycle of steps, in which each transition between steps is determined at a time following a zero crossing of the back emf by a determined delay. This delay is chosen selectively according to the step involved, depending on the duration of a determined one of the preceding intervals between zero crossings.
According to an embodiment of the present invention, the method includes determining, for a motor rotating at constant speed, the duration of the intervals between zero crossings of the back emf, having the longest interval followed by a delay proportional to the duration of the last shortest interval, having the shortest interval followed by a delay proportional to the duration of the last longest interval, and having the intervals of medium duration followed by a delay proportional to the duration of the last interval of medium duration.
According to an embodiment of the present invention, the method consists of having each detection of a zero crossing of the back emf followed by a delay proportional to the duration of a interval shifted from the step under way by a determined number of steps.
According to an embodiment of the present invention, a circuit for controlling a brushless motor includes a switching control circuit providing a voltage to a winding according to a cycle of steps, wherein the cycle includes a transition time between steps, a detection circuit detecting a zero crossing point of a back emf voltage in a winding of the motor, and a timer adding a delay to the transition time, wherein the delay is selected according to a step being executed and wherein the delay is dependent on the duration of one of a preceeding intervals between zero crossing points.